1. Technical Field
The present disclosure relates to a thin film transistor element substrate including an oxide semiconductor, a method of producing the substrate, and an organic EL display device including the thin film transistor element substrate.
2. Description of the Related Art
In recent years, thin film transistor element substrates having channel layers made by using oxide semiconductor materials, such as In—Ga—Zn—O, have been being researched and developed actively. Hereinafter, a thin film transistor element substrate may also be referred to as a TFT substrate. The TFT substrate including an oxide semiconductor material generally has a bottom-gate structure, as in conventional TFT substrates using amorphous silicon. However, top-gate type TFT substrates, which have small parasitic capacitances between a gate electrode and a source electrode and between a gate electrode and a drain electrode and show high performance, have also been being researched and developed actively (Japanese Unexamined Patent Application Publication Nos. 2009-278115 and 2011-228622).
The TFT substrate described in Japanese Unexamined Patent Application Publication No. 2011-228622 will now be described with reference to FIG. 8. The TFT substrate 901 described in the patent document is a top-gate type TFT substrate. The TFT substrate 901 includes a glass substrate 902, an oxide semiconductor layer 903 disposed on the glass substrate 902, and a gate insulating layer 904 and a gate electrode 905 disposed on the channel region 903b located in the center of the oxide semiconductor layer 903. The TFT substrate 901 further includes an aluminum oxide layer 906, an interlayer insulating layer 907, a source electrode 908, and a drain electrode 909. The source electrode 908 is connected to a contact region 903a1 of the oxide semiconductor layer 903 through a contact hole CH1. The drain electrode 909 is connected to a contact region 903a2 of the oxide semiconductor layer 903 through a contact hole CH2.
The contact regions 903a1 and 903a2 on both sides of the channel region 903b in the center of the oxide semiconductor layer 903 must have lower resistances than that of the channel region 903b. Accordingly, an aluminum layer is formed on the oxide semiconductor layer 903, the gate insulating layer 904, and the gate electrode 905 by sputtering. The aluminum layer is then heat-treated to be oxidized into an aluminum oxide layer 906. On this occasion, the contact regions 903a1 and 903a2 are doped with aluminum and have reduced resistances compared to the channel region 903b. Thus, the TFT substrate 901 has contact regions having a reduced resistance with a relatively simple structure. This aluminum oxide layer 906 also has a moisture barrier property.